1. Field of the Invention
The present invention relates to noise elimination circuits in audio applications and, more particularly, to a noise elimination circuit that is adapted for use in the low voltage and small confines of a hearing aid.
2. Description of the Prior Art
Background noise is a problem for many hearing aid wearers. Speech comprehension is normally a difficult task for hearing aid wearers. The volume control of the hearing aid must be set at a level such that average or slightly above average speech levels can be properly comprehended. This volume setting allows any background noise that exists to be amplified and transmitted to the wearer. After a period of time this amplified noise greatly fatigues the wearer, which fatigue further reduces the level of comprehension of speech.
Compression techniques are used which assure that loud sounds do not cause the hearing aid to produce an output which would exceed the wearer's loudness discomfort level, but these techniques do not resolve the problem of noise levels causing fatigue because lower level sounds are still amplified and just higher level sounds compressed.
Low frequency filtering has been used in an attempt to enhance the speech over noise problem but has not been effective. A switch is used which sets the frequency range to a narrower band in one position which is used when the wearer is in a noisy environment and expands the frequency response when set to a second position for use to remove the tinny sounds resulting from the narrow bandwidth when the wearer returns to a quiet environment. This is not a satisfactory solution in many instances because speech comes in a varied number of frequencies, as does noise, and therefore some noise is amplified, some speech is lost and the amplified sounds appear highly artificial.
An alternate technique tried is the compression of low frequencies, in an attempt to remove a large amount of the environmental noise. This did not solve the problem as a significant and fatiguing quantity of noise exists in the middle and high frequency ranges. Neither the narrow bandwidth nor the low frequency solutions dealt with the overamplification problems.